The present invention relates to an improvement in the process for making prills from a melted substance, and more particularly to a process for cleaning the system through which said melted substance passes.
Various processes have been proposed for making prills from highly hydroscopic substances such as urea or from substances which are difficult to handle in powder form, thereby improving the flowability and preventing dust generation. Among these, the principal approach is the melt prilling process, and particularly widely employed is the so-called spray prilling process in which a melted substance is made to flow down or sprayed from the top of a tower by a nozzle with small holes or by centrifugal force with a rotary disc, and prills are obtained either by drying or solidfying the dropping liquid particles through countercurrent gas introduced from the bottom of tower, or by dropping said liquid particles into a medium being inert to said melted substance.
In such prilling process, smooth passage of the melted substance through the nozzle is an important factor, and it is imperative for realizing a continuous operation. The key factors identified for enabling such continuous operation are (1) pre-heating the nozzle in order to prevent viscosity increase of melted substance in the vicinity of the nozzle at the start of the prilling process, (2) quick removal of impurities inevitably contained in the melted substance which give rise to clogging of the nozzle, and (3) applicability of a desirable treatment at the end of the prilling operation, to be explained hereinafter.
For example, during the prilling operation, the nozzle can be partially or totally clogged within a short period by impurities such as dust present in the melted substance or by scale resulting from scorched material. A total clogging of the nozzle will naturally disable the performance of the process, while partial clogging of the nozzle will result in formation of relatively large dripping liquid particles which drop in unsolidified state and stick to the bottom of the tower. The presence of such sticking unsolidified substance will cause reheating, by the heat accumulated therein, of other particles even if they have been completely solidified during the drop, thereby resoftening such particles and thus forming a deposit on the bottom of tower, rendering difficult the discharge of prilled product. It therefore becomes necessary to interrupt the operation and remove the deposit formed on the bottom of the tower. Such deposit, being a defective product, not only reduces the yield of production but also hinders continuous operation. In order to prevent such troubles, it is necessary to provide a strainer in the system between the melting tank and the nozzle for removing the impurities and scale.
On the other hand, upon termination of the operation, in case the melted substance remaining in the system is not removed and left standing, the pathway of melted substance in the prilling apparatus, including the pipes, strainer, pump, spray chamber, nozzle, valves and the like, become clogged by the solidified substance which renders re-start of operation impossible. Also even if the melted substance is removed partially, the remaining melted substance will become deteriorated by the heat of pre-heating to form scales in the system, thus resulting in clogging of the nozzle at the re-start of operation. Further, this step, when repeated, will result in accumulation of a substantial amount of scale in the system, which can only be removed by overhauling the entire system. This phenomenon, therefore, leads to the formation of defective product as explained in the foregoing and hinders continuous operation.
The object of the present invention is to provide a prilling process allowing continuous prill formation economically with a simple apparatus.